Fix append with empty flags array

lib/imap.js

@@ -845,7 +845,8 @@ ImapConnection.prototype.append = function(data, options, cb) {
   if (options.flags) {
     if (!Array.isArray(options.flags))
       options.flags = [options.flags];
-    cmd += " (\\" + options.flags.join(' \\') + ")";
+    if (options.flags.length > 0)
+      cmd += " (\\" + options.flags.join(' \\') + ")";
   }
   if (options.date) {
     if (!isDate(options.date))